Under state law, sanity is determined in a separate phase of the proceedings. The ruling means Sydenham will stay out of prison and will be institutionalized indefinitely, said prosecutor Mary Knox.

"We are gratified that the proper decision was reached by the judge," said Sydenham's attorney, Anne Beles.

Durkin's body was found shortly before noon on Aug. 13, 2012 at his home on the 600 block of Coventry Road, a hillside dwelling with views of the bay.

Sydenham used a serrated, steel steak knife with a 6-inch blade to stab Durkin in the heart, authorities said. Police officers, summoned to the home by a caretaker, found Durkin's body with the knife still lodged in his chest.

Knox said there was "no seemingly rational motive. Defendant and victim had a complicated, somewhat twisted relationship."

Neighbors have described Sydenham and Durkin as companions and said that they were known to have arguments. The slaying sent ripples of shock through Kensington, a wealthy enclave of 5,000 people where violence is rare.

Durkin has been diagnosed for about eight years with Parkinson's disease and helped other patients with the illness.